1. Field of the Invention
The present invention relates to a pneumatic tire. More particularly, the present invention relates to a pneumatic tire having improved steering stability performance on both dry and wet road surfaces.
2. Background Information
With the higher performance levels of vehicles in recent years, there is a stronger demand for a pneumatic tire that can provide stable steering performance on wet road surfaces and stable steering performance on dry road surfaces during high speed travel. In order to improve the steering stability performance of a vehicle on wet road surfaces, pneumatic tires are generally provided with a tread pattern that has many lug grooves and sipes in a tread section to evacuate water efficiently on a wet road surface or ensure the pneumatic tire has grippage on a wet road surface. However, with such a tread pattern, it is difficult to maintain the steering stability performance of the vehicle on dry roads because a mechanical rigidity (tread rigidity) of a land section formed in the tread section declines.
Due to this trade-off relationship, numerous ideas regarding tread patterns have been proposed for achieving good steering stability performance on both drive road surfaces and wet road surfaces. For example, Japanese Laid-Open Patent Application Publication No. 2010-215221 presents a pneumatic tire having four main grooves formed in a region of a tread surface that contacts the ground and five land sections in the tread section, wherein the two center main grooves (of the four) that are positioned toward a tread center are arranged such that a tire equator is disposed between the centers of the two center main grooves and the center of each of the two center main grooves is separated from the tire equator by a distance equal to 8 to 12% of a ground contact width of the tire. The centers of the two main grooves positioned toward both shoulders are each separated from the tire equator by a distance equal to 18 to 32% of a ground contact width of the tire. The groove width of the main groove positioned most toward one side in a widthwise direction of the tire is narrower than the groove widths of the other three main grooves. A ratio Wmax/Wout of a maximum groove width Wmax among the three other main grooves with respect to the groove width Wout of the main groove positioned most toward one side in a widthwise direction of the tire is set to 2.0 to 3.0. Among the five land sections, only the land section positioned most toward one side in a widthwise direction is formed as a string of blocks demarcated with lug grooves and arranged to be separated with a prescribed spacing in a circumferential direction of the tire. The other four land sections are formed as ribs that extend continuously in a circumferential direction of the tire. Additionally, assuming the tire equator is marks a center of the aforementioned ground contact region, a groove area ratio Sin on the tire-widthwise other side of the centerline and a groove area ratio Sout on the aforementioned tire-widthwise one side of the centerline are set such that a ratio Sin/Sout is 1.25 to 1.35, and among the five land sections, each of the three land sections other than the land section positioned farthest toward the other side in the widthwise direction of the tire and the land section positioned farthest toward the one side in the widthwise direction of the tire is configured such that a groove area ratio in the ground contact region of the land section is larger on the tire-widthwise other side of a centerline of the land section than on the tire-widthwise one side of the centerline, the centerline being a circumferential line dividing the land section into two halves separated along a widthwise direction of the tire.
With such a pneumatic tire, the tread rigidity of the land sections in the tread section can be ensured and the steering stability performance on a dry road surface can be improved. Also, since the land section comprising a line of blocks located farthest toward the one side in the widthwise direction of the tire is configured to suppress a decline in rigidity as much as possible, the water evacuation performance can be ensured by the arrangement of the lug grooves and the steering stability performance on wet roads can be improved without causing the steering stability performance on dry roads to decline.